Metal seal rings are commonly used in the hydrocarbon recovery industry to seal between joined tubular members. These seal rings are available in a variety of configurations, such as AX, BX, CX, DX, RX, and VX. These generally have a pair of opposing, conical sealing surfaces for sealing under high contact stress with mating sealing surfaces on the tubular members. The sealing surfaces are energized by drawing the tubular members together under high loads so as to deform the sealing surfaces and cause them to be loaded near or beyond yield of the seal material. The high contact stresses provide a tight seal for sealing high pressure fluid such as oil or gas.
The seal rings, which may also be referred to as “gaskets,” are commonly made from either a “conventional” steel such as low carbon or low alloy steel that lacks corrosion resistance, or a corrosion resistant steel such as stainless steel or nickel-based “corrosion resistant alloy” (CRA). Both the conventional and corrosion resistant steels gaskets have relatively low yield strength, typically on the order of 30–40 ksi. The conventional steel variety, in particular, have a substantially linear thermal expansion coefficient of approximately 6.0E-6 inches/inch/° F., which is generally the same as or similar to that of the surrounding tubular members. The conventional steel gaskets therefore operate satisfactorily over a wide temperature range on the order of 0–350° F. The stainless steel variety are known to have problems, however, when operating over such a wide temperature range, because they typically have a higher thermal expansion coefficient than the surrounding steel members, such as about 7.0E-6 inches/inch/° F.
Because stainless and conventional steels have similar yield strengths, each will yield when energized in an assembly between tubular members. When the assembly is heated, such as from 0 to 350° F., relatively higher expansion will occur in a stainless steel gasket. A stainless steel gasket will therefore typically yield more, all other factors being equal. This does not usually affect the ability of the stainless steel gasket to seal at the higher temperature. Problems arise, however, when the assembly cools. Because the stainless steel contracts more than the surrounding steel of the tubular members, and because the stainless steel has yielded more at the elevated temperature, contact stresses are reduced at the sealing surfaces upon cooling, and a reliable seal may no longer be possible at the lower temperature.
Recent gasket technology is disclosed in U.S. Pat. No. 5,103,915 to Vetco Gray, and U.S. Pat. No. 6,409,176 to Cooper Cameron. Each of these patents disclose tubular assemblies including both primary and secondary sealing surfaces. Secondary sealing surfaces are provided for sealing in case the primary sealing surface becomes damaged. In addition to damaging circumstances such as erosion, another possible way the primary sealing surface can be damaged is if a typical stainless steel gasket is used, having the problems with expansion described above. The above patents, however, offer a fairly expensive solution to the problem, in that the gasket profiles are more complex, having associated costs and difficulty of manufacture.
In the interests of economy and reliability, it is desirable to manufacture both conventional steel and stainless steel gaskets to identical specifications and tolerances, rather than modify the stainless steel gasket to compensate for its elevated temperature characteristics. Furthermore, it is desirable to increase reliability of traditional gaskets having only primary sealing surfaces, rather than having to rely on the more complicated and expensive gaskets having secondary sealing surfaces, such as disclosed in the above patents. Where expensive secondary sealing surfaces do become necessary, it would still be advantageous to allow for consistency in specifications and tolerances between those gaskets using conventional steel and those using stainless steel. An improved sealing ring or gasket is therefore needed having improved reliability and standardization of manufacturing tolerances, especially when operating over wide temperature ranges.